Alteration of kara, “qualities, characteristics”. Originally only found as a suffix. The kara changes to gara as an instance of rendaku (連濁). The rendaku pronunciation was then later used independently.[1]

Cognate with 束 ‎(tsuka, ancient unit of length, equivalent to the amount covered from pinky to forefinger when gripping something; analogous to English hand).[1]

Both likely derive from verb 付く ‎(tsuku, “to set to, to attach, to accompany”), particularly as seen in modern Japanese idioms such as 手を付ける ‎(te o tsukeru, “to set one's hand to something”), 手が付く ‎(te ga tsuku, “to start using something”, literally “one's hand sets on something”).

Ateji or misspelling for uncommon 表外字 ‎(hyōgaiji, “non-standard character”) kanji 枘 ‎(hozo, “navel”, also generic term for something that protrudes slightly). Note that the right half of 柄 is 丙, with a top bar, while the right half of 枘 is 内, without a top bar.[1]